1. Field of the Invention
The present invention relates to an improved chemical vapor deposition method which enables one to form various deposited films such as metal films, semiconductor films and insulating films usable in the preparation of semiconductor devices, memories such as photoelectromagnetic diskes, flat panel displays, and the like using a liquid film-forming raw material. The present invention also relates to an apparatus suitable for practicing said chemical vapor deposition method.
2. Related Background Art
There are known a number of deposited films formed by a chemical vapor deposition technique using an appropriate chemical vapor deposition apparatus. These deposited films can be categorized into three groups i.e., metal film group, semiconductor film group and insulating film group.
Any of these deposited films is required to be formed in such manner that it satisfies the related conditions in order for it to be practically applicable as a constituent element of the device. Particularly, in the case of forming a semiconductor deposited film by a chemical vapor deposition method (hereinafter referred to as CVD method), it is desired to be formed in such a state that it is free of defects and uniform in thickness.
Similarly, in the case of forming an insulating deposited film by a CVD method, it is desired to be formed in such a state that it is free of defects, uniform in thickness and excels in step coverage. There is a specific requirement relative to the step coverage in the case of forming an insulating deposited film since the insulating deposited film is often used for the purpose of insulating between wirings or protecting an uneven surface of a constituent member of electronic devices such as integrated circuit (IC).
Further, in the case of forming a metal deposited film by a CVD method, it is desired to be formed in a similar state to that of the insulating deposited film, that is, in such a state that it is uniform in thickness and excels in step coverage. The metal deposited film is often used for the purpose of wiring ICs. In this case, the metal deposited film is formed at least to satisfy the condition relative to step coverage because the upper and lower wirings are connected by the metal deposited film formed through apertures which are so-called contact-holes or through-holes.
There are known various chemical vapor deposition apparatus (the chemical vapor deposition apparatus will be hereinafter referred to as CVD apparatus) capable of practicing the above CVD methods.
A typical example of such CVD apparatus is of the constitution schematically shown in FIG. 8. Explanation will be made of the CVD apparatus shown in FIG. 8.
In FIG. 8, numeral reference 403 indicates a reaction vessel comprising a tube made of quartz or the like. The reaction vessel 403 contains a plurality of substrate holders 410 installed therein. Numeral reference 409 indicates a substrate on which a film is to be deposited which is placed on each of the substrate holders 410.
Numeral reference 408 indicates an exhaust pipe which is connected to a main vacuum pump 404 comprising a mechanical booster pump or the like and a sub-vacuum pump 405 comprising a rotary pump or the like. The inside of the reaction vessel 403 can be evacuated through the exhaust pipe 408 by actuating these vacuum pumps.
The reaction vessel 403 is provided with a gas supply system comprising a gas feed pipe 407 extending from a bomb (a bubbling vessel in other words) 402 which is equipped with a bubbling mechanism capable of bubbling a liquid raw material 411 such as an organic aluminum contained in the bomb 402. Numeral reference 406 indicates a gas supply pipe for introducing a bubbling carrier gas from a gas reservoir (not shown) into the bomb 402. The gas supply pipe is provided with a controller 401 capable of controlling the flow rate of the bubbling carrier gas to be introduced into the bomb 402.
The liquid raw material contained in the bomb 402 is bubbled by the action of the bubbling carrier gas introduced therein through the gas supply pipe 406 to produce a gaseous mixture comprising the raw material and the carrier gas, which is followed by supplying into the reaction vessel 403 through the gas feed pipe 407.
The CVD apparatus shown in FIG. 8 is acceptable as long as film formation is conducted on a laboratory scale by a conventional CVD technique. However, the CVD apparatus is not suitable in the case where a deposited film is formed while minute-processing or in the case where a large area deposited film is formed.
Description will be made of this situation of the conventional CVD apparatus shown in FIG. 8.
Incidentally, in recent years, the use of an aluminum film formed not by means of a sputtering method but by means of a CVD method as a wiring material in semiconductor devices of a very large scale integration such as VLSI, ULSI, etc. has been highlighted. Particularly, there have been reported experimental results in that in the CVD method using an organic compound comprising an organic aluminum as a film-forming raw material, the conditions for depositing an aluminum film on an insulating material are distinguishably different from those for depositing said aluminum film on a conductive material, and because of this, said CVD method makes it possible to deposit an aluminum film selectively only on a conductive material or a semiconductor material (see, T. Shinzawa et als., "SELECTIVE Al CVD USING DIMETHYLALUMINUM HYDRIDE", pp. 377-382, Mat. Res. Soc. Symp. Proc. VLSI V, 1990; and T. Amazawa et al., "A 0.25 .mu.m VIA PLUG PROCESS USING SELECTIVE CVD ALUMINUM FOR MULTILEVEL INNTERCONNECTION", pp. 265-266, IEDM 1991).
This CVD method of depositing an aluminum film selectively on an object is considered to be useful in the case of preparing a minute integrated circuit. Especially in the case where the aspect ratio of an aperture (that is, the ratio between the depth and the diameter of the aperture) exceeds 1 upon preparing such minute integrated circuit, the precise wiring using an aluminum film could be attained by the above CVD method, although it cannot be attained as desired by a sputtering method.
In the case where wiring with the use of an aluminum film upon preparing the minute integrated circuit wherein the aspect ratio is beyond 1 is conducted by means of a sputtering technique, it is extremely difficult to accomplish the wiring as desired without causing any disconnection. The reason for this will be described with reference to FIGS. 9(a) and 9(b) respectively showing a stacked configuration obtained by means of a sputtering technique. In FIGS. 9(a) and 9(b), numeral reference 201 indicates a single crystal silicon substrate, numeral reference 202 indicates an insulating layer composed of silicon dioxide or the like, and numeral reference 203 indicates a wiring material such as aluminum or the like. Numeral reference 204 in FIG. 9(a) indicates a recession occurred at the wiring material 203 situated in an aperture of the insulating layer 202, wherein the recession becomes to cause a disconnection of the wiring. Numeral reference 205 in FIG. 9(b) indicates a cavity occurred within the wiring material 203 situated in an aperture of the insulating layer 202, wherein the cavity becomes to cause a disconnection of the wiring.
FIG. 9(a) shows a state of wiring wherein the aspect ratio is relatively small, that is, less than 1. FIG. 9(b) shows a state of wiring wherein the aspect ratio exceeds 1.
In each of the two stacked configurations shown in FIGS. 9(a) and 9(b), all the constituents except the single crystal silicon substrate 201 are formed by means of a sputtering technique.
On the other hand, FIG. 9(c) is of a stacked configuration formed utilizing the selective deposition by the CVD method.
In FIG. 9(c), numeral reference 301 indicates a single crystal silicon substrate, numeral reference 302 indicates an insulating layer composed of silicon dioxide or the like, numeral reference 303 a metal material formed by means of a CVD technique, numeral reference 304 indicates an aluminium wiring formed by either a sputtering technique or a CVD technique.
Now, it is understood that in each of the two stacked configurations shown in FIGS. 9(a) and 9(b) formed by the sputtering technique, there are unavoidably occurs the recession 204 or the cavity 205 at or within the aluminum wiring 203. On the other hand, as apparent from what shown in FIG. 9 showing a stacked configuration formed utilizing the selective deposition by the CVD method, it is understood that a disconnection of the wiring hardly occurs since the aperture of the insulating layer 302 is filled up with the aluminum wiring material 303 formed by the selective deposition by the CVD method.
The aluminum film selective deposition by the CVD method according to the above report can be practiced using the CVD apparatus shown in FIG. 8, for example, in the following manner.
That is, a carrier gas CGS such as H.sub.2 gas is supplied into the bomb 402 (the bubbling vessel in other words) containing a liquid organic aluminum 411 therein through the gas supply pipe 406 while controlling the flow rate of the carrier gas by the gas flow controller 401. Specific examples of the organic aluminum are dimethylaluminum hydride (DMAH), triisobutylaluminum (TIBA), etc. which are in the liquid state at normal temperature. When the carrier gas is introduced into such liquid organic aluminum 411 contained in the bomb 402, the liquid organic aluminum is bubbled by the action of the carrier gas to produce a gaseous mixture comprising the organic aluminum and the carrier gas, which is followed by flowing into the reaction vessel 403 through the gas feed pipe 407. The gaseous mixture into the reaction vessel 403 is decomposed by the action of heat radiated from each of the substrates 409 maintained at elevated temperature to cause chemical reaction with the surface of each of the substrates, whereby an aluminum film is deposited on each of the substrates 409.
In this case, the gaseous mixture unreacted in the reaction vessel 403 is exhausted through the exhaust pipe 408 by operating the main vacuum pump 404 and the sub-vacuum pump 405.
The above aluminum film selective deposition by the CVD method using the CVD apparatus shown in FIG. 8 seems effective as long as the aluminum film selective deposition is conducted on a laboratory scale by a conventional CVD technique. However, the foregoing effects of causing selective deposition of an aluminum film on a given object according to the above report cannot be attained as desired in the case where the CVD apparatus is structurally modified or it is enlarged to an industrial scale, wherein problems entail in that the aluminum film selective deposition does not proceeds as desired, defects are increased not only in a metal film but also in a semiconductor film, and a desirable step coverage is hardly attained in an insulating film. The reasons for this are considered due to the factors that (i) the mixing ratio between a liquid compound as the raw material and a gas as the carrier gas is hardly controlled as desired, (ii) the mixing ratio of the liquid compound is varied depending upon a change in the temperature in the vicinity of the bubbling vessel, and (iii) the mixing ratio of the liquid compound is varied depending upon the amount of the unvaporized residue in the bubbling vessel.